Ultrasonic method and apparaus for determination of vessel location and size

ABSTRACT

An improved technique for establishing the position, dimensions and orientation of a vessel which is essentially free of internal acoustic discontinuities involves scanning the vessel with a beam of ultrasonic energy and, from the ultrasonic echoes received in the A-mode lines of sight of the ultrasonic beam, establishing a B-mode image of the vessel. An angle marker, in the form of a short line segment, is established, using the B-mode image, on or near a straight axis of the vessel. The length of the angle marker is divided into a number of sub-sectors, each of which receives approximately the same number of A-mode lines of sight of the ultrasonic beam. An average A-mode signal is obtained for each sub-sector. Comparison of the average A-mode signals provides information about the orientation of the angle marker relative to the axis of the vessel. Combining the average A-mode signals to obtain a composite signal and performing edge-detection analysis of the composite signal enables the edges of the vessel to be established with precision, and the diameter of the vessel to be determined accurately. A single computer can programmed to perform the analysis and re-position the angle markers (if necessary) on to the axis of the vessel. Using information obtained from this technique, accurate values of the flow rate of a fluid moving within the vessel can be obtained when Doppler frequency shift measurements are made.

TECHNICAL FIELD

This invention relates to the measurement of the size and orientation ofhidden vessels, such as the vessels in the human body through whichblood flows, using ultrasonic echoscopy. It has particular applicationto the measurement of the flow of blood in blood vessels using theDoppler frequency shift in conjunction with ultrasonic measurementtechniques, but it is not limited to blood flow measurement. The presentinvention provides a method for improving the current ultrasonictechniques for measuring the size and orientation of vessels, and hencefor obtaining measurements of the flow through such vessels.

BACKGROUND

It is now well known that ultrasonic echoscopy techniques can be used toprovide information about an object that is not visible to the eye. Thebasic technique of ultrasonic echoscopy involves directing a short pulseof ultrasonic energy, typically in the frequency range from 1 MHz to 30MHz, into the region of the object that is being examined, and observingthe energy that is reflected, as an echo, from each acoustic impedancediscontinuity in that region. Each echo received is converted into anelectrical signal and displayed as either a blip or an intensified spoton a single trace of a cathode ray tube or television screen. Such adisplay of the echoes is known as an "A-mode" echograph or echogram, andis useful in a number of diagnostic techniques to locate the boundariesof the object or to provide other information about the region intowhich the pulse of ultrasonic energy has been directed.

If a series of adjacent A-mode displays are obtained (for example, byphysically or electrically moving the transmitting transducer whichproduces the pulses of ultrasonic energy, or by scanning the directionof transmission of the pulses of ultrasonic energy), a two-dimensionalimage of the object under examination may be displayed on the cathoderay tube or television screen. Such an image or display of acousticdiscontinuities is known as a "B-mode" image or display.

The use of the Doppler frequency shift in the ultrasonic examination offlowing liquids and moving objects is also well known. Many echoscopeswhich perform the B-mode imaging examination described above can alsoperform Doppler frequency shift measurements in respect of echoesreturned from moving objects within the region receiving ultrasonicenergy from the echoscope When the object under examination is a bloodvessel, measurement of the Doppler shift of echoes from the blood cellswithin the vessel permits the velocity of those blood cells to beestimated. As pointed out by R W Gill, in his article entitled"Measurement of Blood Flow by Ultrasound: Accuracy and Sources ofError", which was published in Ultrasound in Medicine and Biology,Volume 11 (1985), pages 625 to 641, it is possible to measure the totalvolume of flow per unit time using an ultrasonic examination techniquewhich includes the measurement of frequency changes due to the Dopplereffect.

In ultrasonic examinations including Doppler frequency shiftmeasurements, it is necessary to obtain echoes from a limited volume ofthe flowing liquid which is within the vessel being examined. This isachieved by fixing the line of sight of the ultrasonic transducer and,in the most commonly used version of Doppler measurement known as"pulsed Doppler", analysing the echoes obtained from the sample volumefor a limited range of time delays. The Doppler shift in the receivedechoes is averaged in order to calculate the average speed of scatterersin the flowing liquid.

In current applications of the pulsed Doppler technique, a small samplevolume within the vessel is selected by the operator of the echoscope,who moves a graphical representation of the sample volume over a B-modeimage of the vessel. In this way, the B-mode imaging equipment is usedto steer the ultrasonic beam and adjust the sample volume delay so thatthe actual sample volume position corresponds to that part of the vesselwhich is to be the subject of the Doppler shift measurement. Theorientation of the vessel has to be known, so that the velocity of theliquid within the vessel may be calculated from the well-known Dopplerequation: ##EQU1## where f_(D) is the Doppler shift frequency, f_(o) isthe transmitted frequency, v is the blood velocity, c is the speed ofsound and θ is the angle between the line of sight of the ultrasonicbeam and the direction of flow of the liquid. In current implementationsof this technique, the orientation of the vessel is obtained fromobservations of the graphical representation of the sample volume in theB-mode ultrasonic image.

In the volumetric measurement of flow, a larger sample volume is placedto encompass the entire vessel, and the total flow is calculated usingthe relationship: ##EQU2## where d is the vessel diameter and f_(D) isthe mean Doppler shift in frequency. When applying this formula, thediameter of the vessel is estimated by the operator, who identifies thepositions of the two internal vessel walls on the B-mode image andplaces cursors on their images. The diameter of the vessel is taken asthe distance between the cursor positions. This is a difficultmeasurement, and because the flow is directly proportional to the squareof the diameter in the expression for flow, errors in the diametermeasurement translate into greater errors when the flow values areestimated.

Another factor affecting the accuracy of blood flow measurements is thefact that, in humans and animals, the diameter of most vessels variesduring the cardiac cycle. This is particularly so in the case ofarteries. Hence, for greatest accuracy, the instantaneous values off_(D) and d should be obtained repeatedly and the expression in equation(2) should be averaged over several cardiac cycles.

DISCLOSURE OF THE PRESENT INVENTION

It is an object of the present invention to provide an improvedultrasonic echoscopy method and apparatus in which the following threepieces of information may be obtained in real time:

(a) vessel location for accurate placement of the sample volume;

(b) vessel orientation (for the "cos θ" factor correction if liquid flowmeasurements are to be made); and

(c) vessel diameter (for volumetric flow measurements).

Preferably, utilising the present invention, the measurement process canbe repeated frequently if any of these parameters are likely to changewith movement (for example, in the cardiac cycle).

Automatic adjustment of the position of the sample volume in ultrasonicechoscopy has been demonstrated previously. One technique for doing thisis described in the paper by J G Davis, K L Richards and D Greeneentitled "A sample volume tracking unit for pulsed Dopplerechocardiography", which was published in IEEE Transactions onBiomedical Engineering, volume BME-26 (1979) pages 285-288. However,that technique uses a single line of sight and thus is capable of axialshifts only. That and similar prior art techniques do not recogniseactual structures associated with the surrounding tissues, nor do theypermit measurement of the vessel orientation and diameter.

The present invention achieves computational efficiency by usingone-dimensional signal processing techniques. It makes use oftwo-dimensional data from a selected number of adjacent A-modeultrasonic lines of sight in a given B-mode image frame and iteratesusing data from successive images. It uses echoes obtained while theechoscope is in its imaging mode, and makes use of the facts that (i)images of vessels containing a fluid (for example, blood vessels) arenormally free of internal echoes, and (ii) the vessels on which Dopplerstudies are carried out, or in respect of which other measurements arelikely to be taken, are normally locally straight.

According to the present invention, there is provided a method ofestablishing the dimensions and orientation of a vessel which, whensubjected to ultrasonic echoscopy examination, is found to beessentially free of substantial internal acoustic discontinuities, saidvessel having an axis which is straight in at least part of the vessel,said method comprising the steps of

(a) obtaining a B-mode ultrasonic echogram image of said vessel or thepart thereof in which said axis is straight from a number of A-modeultrasonic images, each corresponding to a respective A-mode line ofsight of a beam of ultrasonic energy which is scanned over said vesselor said part thereof;

(b) establishing an angle marker in the form of a short line segmentwithin or close to the B-mode image of said vessel or said part thereof;

(c) establishing a plurality of adjacent sub-sectors within the B-modeechogram image of said vessel or part thereof; each of said sub-sectorsbeing intersected by said angle marker; each sub-sector receivingsubstantially the same number of adjacent A-mode lines of sight of saidscanning beam;

(d) obtaining an average video-detected A-mode signal for each of saidsub-sectors,

(e) performing a comparison of said average A-mode signals of thesub-sectors and determining from said comparison the difference betweenthe orientation of the axis of said vessel and said angle marker and theorientation of said axis of said vessel;

(f) combining said average A-mode signals to obtain a composite A-modesignal for said vessel or part thereof; and

(g) performing edge-detection analysis on said composite A-mode signal,to obtain a value of the diameter of said vessel or part thereof.

This method, it will be apparent, may be combined with the Dopplermeasurement technique to obtain an accurate measurement of the flow of aliquid within the vessel (for example, the flow of blood in an artery,or through a cardiac vessel).

Preferably, the data obtained from the scanning beam of ultrasonicenergy is obtained in digital form, so that the production of theaverage A-mode signal for each sub-sector, the comparison (typicallyobtained by signal superimposition) of such average A-mode signals, thedifference determinations, the production of a composite A-mode signal,and the edge-detection analysis may be performed using (a) digitalcomputer, appropriately programmed.

The present invention also encompasses apparatus for establishing theorientation and cross-sectional dimensions of a vessel having an axis,said apparatus comprising:

(a) conventional apparatus for generating a beam of ultrasonic energyand scanning said beam over at least part of said vessel, to obtain aB-mode ultrasonic echogram image of said vessel or part thereof from anumber of A-mode ultrasonic images, each corresponding to a respectiveA-mode line of sight of said beam;

(b) operator-activated means for establishing an angle marker in theform of a short line segment within or close to the B-mode image of saidvessel or part thereof;

(c) accumulation means for accumulating echoes relating to respectivesub-sectors of a plurality of sub-sectors of said B-mode image, saidsub-sectors (i) being adjacent to each other, (ii) being intersected bysaid angle marker, and (iii) containing substantially equal numbers ofadjacent A-mode lines of sight of said scanning beam;

(d) averaging means for producing a respective average A-mode signal foreach of said sub-sectors;

(e) comparison means for comparing said average A-mode signals and forobtaining differences between the average A-mode signals, to enable theorientation of said axis relative to said angle marker to be computed;

(f) combining means for combining said average A-mode signals to producea composite A-mode signal; and

(g) analytical means to perform edge analysis on said composite A-modesignal and to obtain therefrom a value of the diameter of said vessel orpart thereof.

As will be apparent from the comments made above concerning the methodof the present invention, Doppler signal-generating and processingequipment may be used in conjunction with this apparatus forestablishing the orientation and cross-sectional dimensions of thevessel, to provide apparatus for accurately measuring the flow of liquidwithin the vessel.

These and other features of the present invention will be demonstratedin the following description of an embodiment of the present invention,which is provided by way of example only. In the following description,reference will be made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a flow measurement system incorporating thepresent invention and a Doppler frequency shift measurement arrangement,with the ultrasonic beam scanning arrangement indicated schematicallythereon.

FIGS. 2A and 2B illustrate the creation of sub-sectors and containrepresentations of the average A-mode signals obtained from thesub-sectors.

FIG. 3 shows waveforms used in part of the edge detection analysis.

FIG. 4 shows another waveform and its processing in accordance with thepreferred edge detection algorithm.

FIGS. 5A and 5B are diagrams which illustrate a situation in which theangle marker will be caused to be displaced along its length.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

The apparatus illustrated in FIG. 1 includes a conventional B-modeultrasonic echograph arrangement comprising an ultrasonic beamgenerating, beam transmitting and echo receiving transducer 10, which isarranged to scan the ultrasonic beam to and fro through a sector 11. Aportion of a tubular vessel 13 (for example, an artery) that is to beinvestigated is located within the sector 11. Echoes received fromacoustic discontinuities in the path of the lines of sight of thescanned ultrasonic beam are processed in a conventional manner by a timegain compensation (TGC) processing unit, non-linear amplification, andvideo detection stages shown collectively as unit 17 in FIG. 1. TheB-mode image of the region of sector 11 is displayed in a conventionalmanner (on the screen 19 of a television set or cathode rayoscilloscope) by the imaging display unit 18.

A short, linear angle marker is located in the B-mode image of thesector 11 in a location corresponding to the line 15 shown in the sector11. Thus, effectively, the operator of the apparatus establishes anangle marker 15 within a volume 14 of the vessel that is to beinvestigated. It will be appreciated that the placing of the anglemarker assumes knowledge of the position of the vessel 13. If thatknowledge is not available (for example, if the B-mode image is notentirely clear and the vessel 13 cannot be conclusively identified fromthe image), the angle marker may be placed by an intelligent guess ofthe location of the vessel 13. Using the present invention, it will bepossible to progressively re-position the angle marker by an iterativeprocess until it lies along the axis of the vessel 13. A similarre-positioning of the angle marker will be obtained if the angle markeris initially alongside or intersecting the vessel 13.

Note that the positioning of the angle marker assumes that the vessel 13is locally straight.

In the illustrated embodiment, as shown in FIG. 2, the portion of thevessel 13 that is being investigated is divided into four sub-sectors,referenced 23, 24, 25 and 26. It will be appreciated, however, that anypractical number of sub-sectors may be adopted. Each sub-sector isintersected by the angle marker 15. Each sub-sector is chosen so that itcontains substantially the same number of individual A-mode lines ofsight of the scanned beam of ultrasonic energy.

The individual A-mode image signals obtained for each of thesub-sectors, in the region where it is expected (or known) that thevessel 13 is located (that is, between the dashed lines 27 and 28 ofFIG. 2A, which are parallel to the angle marker line 15), are thenaccumulated and an average A-mode video image signal is generated foreach sub-sector. In FIG. 2, the average A-mode signal for the sub-sector23, obtained from echoes received from the region between the lines 27and 28, is shown as waveform 32. The corresponding average A-mode videoimage signals for the sub-sectors 24, 25 and 26 are shown as signalwaveforms 31, 30 and 29, respectively.

The waveforms 29, 30, 31 and 32 are displayed in FIG. 2B with the axisof the time of arrival of echoes, relative to the intersection of theangle marker 15, horizontal. It will be seen that although (as expected)the waveforms 29, 30, 31 and 32 have similar shapes, they are displacedhorizontally and progressively with respect to each other. This apparentoffset occurs because the angle marker does not lie parallel to the axisof the vessel 13. The angle that the corresponding points on thedisplayed waveforms present relative to the angle marker indicates theangle between the angle marker and the axis of the vessel. This anglecan be used to determine the orientation of the vessel 13 and/or a newposition for the angle marker 15 which is parallel to the axis of thevessel 13.

It will also be apparent that each of the waveforms 29, 30, 31 and 32contains low-echogenic regions between the (parallel) lines 33 and 34,showing that the region between the lines 33 and 34 is where the vesselis located. The actual position of the intersects between the averageA-mode lines of sight signals (assumed to be obtained on the mid-line ofeach sub-sector) and the vessel walls is then obtained. Preferably thisis done using the edge locating algorithm described below with referenceto the composite line of sight for the sector. The location of the axisof the vessel 13 relative to the angle marker is then determined(typically obtained by fitting a least squares regression straight linewith position along the angle marker as independent variable anddeviation of midline position as dependent variable). This informationwill enable the angle marker to be repositioned on substantially theaxis of the vessel before the next measurement of the vessel 13 isundertaken.

Although the information obtained from the waveforms 29, 30, 31 and 32is adequate to modify the inclination and range of the angle marker, andmay be used to estimate the positions of the edges of the vessel 13, thedesirability of having a very accurate determination of vessel sizemeans that usually a more detailed analysis is performed to determinethe location of the edges of the vessel 13 with precision.

A number of algorithms have been established to determine accurately theactual positions of the walls of the vessel 13. The preferred algorithm,a one-dimensional edge detection method, is described below. As notedabove, this algorithm is also the one preferred when determining theorientation of the vessel, using the average A-mode line of sightsignals for each sub-sector.

The average A-mode signals for each of the sub-sectors (that is,waveforms 29, 30, 31 and 32) are combined to form a global or compositesingle A-mode signal, shown as waveform 41 of FIG. 3.

With regard to the generation of the combined (or global, or composite)waveform 41 of FIG. 3, it will be appreciated that if the systemillustrated in FIG. 1 is used, it will be necessary to put in acorrection factor when combining the average A-mode signals, becausethey are not parallel to each other. This is preferably done bydigitally "re-sampling" the signal waveforms to produce the signalwaveforms that would be received if each average A-mode signal wasobtained from a beam of ultrasonic energy which crosses the vessel atright angles to the axis of the vessel. It will also be appreciated thatif, instead of using an ultrasonic transducer which scans a beam througha sector, a linear array transducer is used, so that all of the A-modelines of sight of the scanned beam are parallel to each other, acorrection will still be required unless the lines of sight intersectthe vessel axis at right angles. (When producing the average A-modesignals for each sub-sector, as discussed above, using the equipment ofFIG. 1, it is not necessary to compensate for the change in angle,relative to the axis of the vessel, for each line of sight in asub-sector.

The corrected "raw data" waveform 41 is smoothed using a Gaussian filter(a suitable degree of smoothing corresponds to a Gaussian standarddeviation of 1.1 mm) and the resulting smoothed curve 42 is inspected tofind its minimum 43. The minimum 43 occurs at the location of the axisof the vessel 13. The smoothed curve 42 is differentiated by the methodof first differences and the result (waveform or curve 44) is analysedto detect a minimum 45, which corresponds to the vessel wall nearest tothe ultrasonic transducer, and a maximum 47, which corresponds to thevessel wall farthest away from the ultrasonic transducer.

The edge detection algorithm is required to have the further capabilityof distinguishing the vessel wall from spurious echoes internal to thevessel, or large specular reflectors located outside it. One method ofachieving this is as follows.

In FIG. 3, using the position corresponding to the minimum 43 of thesmoothed data curve 42 as a starting point, a search is performed in thedifferentiated data curve 44 for minima closer to the transducer thanthe starting point, and for maxima further from the transducer than thestarting point. Within a certain distance from the starting point(chosen arbitrarily, but realistically, such as 12 mm), the largestnearer local minimum and largest farther local maximum are found. Asecond search then locates the respective nearer local minimum andfarther local maximum nearest the starting point and exceeding asuitable fraction (chosen in the light of experience and typically about0.25) of the local minimum and maximum found earlier. The nearer localminimum and farther local maximum nearest the starting point, located bythis second search, are chosen as the minimum and maximum correspondingthe wall positions of the vessel 13.

This selection technique is further illustrated by FIG. 4. In FIG. 4, aplot of the smoothed, differentiated data 51 is shown, with the minimumof the undifferentiated data marked 52. The plot 51 also shows a minimumvalue and three local maxima 53, 54 and 55, which are candidates forinterpretation as the far vessel wall, are shown. After measuring theheight of the largest local maximum 55, a threshold level 56 at asuitable fraction (for example, 0.25 of the height of the maximum 55) isestablished. The local maximum 54 nearest the starting point whichexceeds this threshold is taken as the wall position.

After determining the positions of the walls of the vessel 13, the anglemarker is normally repositioned to lie substantially along the axis ofthe vessel, and the measurements are repeated.

FIG. 5 illustrates the circumstances in which movement of the anglemarker along its length may be invoked, namely, when the diameter of thevessel, as displayed in the image of the vessel, appears to tapernon-linearly. A linear tapering of the vessel presents no problem, butif the image of the vessel tapers non-linearly, this indicates that theimage is not entirely along the major axis of the vessel.

In FIG. 5A, the wall positions 61 and 62 of a vessel have beendetermined using an angle marker 63 (shown as a dashed line). That theimage of the vessel is tapered non-linearly is evident from thediameters 64, 65, 66 and 67, derived from the average A-mode videodisplay signals of the sub-sectors 68, 69, 70 and 71, which have beenplotted as part of FIG. 5B, If one of the extreme diameters 64 or 67 isless than a predetermined fraction (such as 0.9) of that predicted byleast squares regression on the other three diameters, the angle markeris moved lengthwise to a new position 72 which is substantially on theaxis of the vessel, but is farther away from the smallest diameter ofthe vessel, by a suitable distance (such as one quarter of its length).The length of the angle marker, which defines the length of the regionof the vessel 13 that is being investigated, will normally remainconstant.

From the foregoing description, it will be seen that, in itsimplementation, the present invention records and accumulates data fromA-mode images obtained using a scanning (that is, an imaging)transducer. Preferably the data is in digital form. This data isrecorded after passing through the time gain compensation, non-linearamplification and video detection stages of conventional ultrasoundprocessing. Only A-mode images from lines of sight which pass throughthe angle marker are required to be recorded. The part of the scannedbeam of ultrasonic energy which intersects the angle marker is dividedinto a number of parts (called "sub-sectors", typically four in number),each containing an approximately equal number of lines of sight of theultrasonic beam. Within each sub-sector, a sum is formed of all of thelines of sight within that sub-sector to produce a single A-mode videoimage signal for that sub-sector. In accumulating the summation, thetime delay of the beginning of the data from each line of sight isadjusted so that the mid-point of each line of sight coincides with theangle marker, thus taking into account the assumed position andorientation of the vessel.

The accumulated data segment from each sub-sector is analysed using anedge-detection algorithm, to determine the location of the near and farvessel walls at the centre of the sub-sector. The vessel midline or axisis assumed to be halfway between the wall positions determined for eachsub-sector, measured along the appropriate line of sight (usually themiddle line of sight of each sub-sector). The measured locations for theseveral midline points (one for each sub-sector) are compared with thelocation of the angle marker and a least squares fit is used tocalculate the error in range and angulation of the vessel relative tothe angle marker.

The data segments for each sub-sector are then added together, applyingthe appropriate correction if the lines of sight do not intersect thevessel at right angles to its axis, and the wall detection algorithm isapplied to their sum. The distance between wall positions in this lastcalculation gives the measured vessel diameter. The errors in range andangulation (determined using the least squares regression methodmentioned above) are converted into range and orientation correctionsfor the angle marker, which are used to establish a new angle markerposition. The length of the angle marker remains constant, and itscentre normally remains on the same line of sight. However, if the imageof the vessel indicates that its diameter tapers non-linearly, the anglemarker may be moved lengthwise to its new position.

The new angle marker position is used as input data for the nextiteration of the vessel tracking algorithm, which occurs during asubsequent image scan. If the initial angle marker position is greatlyin error, several iterations (one per imaging scan) of the algorithm maybe necessary to stabilise to the correct value. In a possibleimplementation, the operator places a single cursor within the vessel ofinterest, and an angle marker of fixed length and orientation (e.g.horizontal) is generated as the initial angle marker, which iterativelyadjusts itself to the true vessel orientation.

Those skilled in the art of ultrasonic echoscopy will recognise that aparticular implementation of the invention is in conjunction with theknown ultrasonic imaging and pulsed Doppler functions, as shown inFIG. 1. These functions may use the same transducer, differenttransducers, or different parts of the same transducer where thattransducer is of an array type. The imaging transducer may alternate itslines of sight with the Doppler lines of sight, or it may assemble acomplete image during a break in the Doppler data acquisition.

The vessel orientation information is supplied to the Doppler module ofthe machine for substitution in equation 1, and so provide correctlyscaled velocities of the liquid in the vessel 13. In machines whichmeasure volumetric flow, the orientation θ and diameter d aresubstituted in equation 2 to determine flow. The position of the samplevolume is adjusted for subsequent Doppler measurement, and the positionand orientation of the angle marker become the input values for the nextiteration of the tracking algorithm.

INDUSTRIAL APPLICABILITY

Three particularly useful applications of the present invention are asfollows:

(a) In studies of the velocity distribution of blood in vessels over thecardiac cycle, when the sample volume is usually smaller than thevessel, this invention will allow the sample volume to be maintained ina fixed position relative to the (possibly) moving vessel, and willautomatically calculate cos θ for calculating velocity using equation 1.

(b) In volumetric flow studies, when the sample volume is usually largerthan the vessel, the invention will enable the sample volume to bemaintained in a fixed position relative to the (possibly) moving vessel,and will automatically calculate cos θ and the vessel diameter d forcalculating flow using equation 2.

(c) In Doppler colour flow studies, a two-dimensional image is colouredaccording to the local velocity of flow, as described by K. Miyatake, M.Okamoto, N. Kinoshita, S. Izumi, M. Owa, S. Takao, H. Sakakibara and Y.Nimura in their article entitled "Clinical applications of a new type ofreal-time two-dimensional flow imaging system", which was published inAmerican Journal of Cardiology, volume 54 (1984), pages 857-868. Thepresent invention will allow the use of equation 1 to correct thedisplayed velocity so that the displayed velocity is the actual speed ofblood along the vessel, rather than the component along the line ofsight.

This list of particularly useful applications of the present inventionis not intended to be exhaustive.

Finally, it should be noted that although a specific implementation andapplication of the present invention has been illustrated and describedabove, the present invention is not limited to that implementation andapplication. Variations of and modifications to the present inventionmay be made without departing from the present inventive concept.

We claim:
 1. A method of establishing the dimensions and orientation ofa vessel which, when subjected to ultrasonic echoscopy examination, isfound to be essentially free of substantial internal acousticdiscontinuities, said vessel having an axis which is straight in leastpart of the vessel, said method comprising the steps of(a) obtaining aB-mode ultrasonic echogram image of said vessel or the part thereof inwhich said axis is straight from a number of A-mode ultrasonic images,each corresponding to a respective A-mode line of sight of a beam ofultrasonic energy which is scanned over said vessel or said partthereof; (b) establishing an angle marker in the form of a short linesegment within or close to the B-mode image of said vessel or said partthereof; (c) establishing a plurality of adjacent sub-sectors within theB-mode echogram image of said vessel or part thereof; each of saidsub-sectors being intersected by said angle marker; each sub-sectorreceiving substantially the same number of adjacent A-mode lines ofsight of said scanning beam; (d) obtaining an average video-detectedA-mode signal for each of said sub-sectors, (e) performing a comparisonof said average A-mode signals of the sub-sectors and determining fromsaid comparison the difference between the orientation of the axis ofsaid vessel and said angle marker and the orientation of said axis ofsaid vessel; (f) combining said average A-mode signals to obtain acomposite A-mode signal for said vessel or part thereof; and (g)performing edge-detection analysis on said composite A-mode signal, toobtain a value of the diameter of said vessel or part thereof.
 2. Amethod as defined in claim 1, in which the step of performingedge-detection analysis includes the sub-steps of(i) smoothing thewaveform of the composite A-mode signal and identifying a minimum valuetherein which corresponds to the position of the axis of said vessel;(ii) differentiating the smoothed waveform of the composite A-modesignal; (iii) identifying a minimum in the differentiated waveform,which occurs between the generation point of the beam of ultrasonicenergy and the axis of said vessel and which corresponds to one wall ofthe vessel and identifying a maximum in the differentiated waveform,farther from generation point of the beam of ultrasonic energy than theaxis of said vessel, which corresponds to the other wall of the vessel;and (iv) adopting the distance between the minimum identified in step(iii) and the maximum identified in step (iii) as the diameter of thevessel.
 3. A method as defined in claim 2, including the additional stepof repeating steps (a) to (g) and using the information obtained fromstep (e) of the last preceding sequence of steps (a) to (g) to performthe step of establishing the angle marker so that the angle marker ispositioned substantially on said axis of said vessel.
 4. A method asdefined in claim 3, in which, when step (e) provides information showingthat the B-mode image of said vessel indicates that said vessel isnon-linearly tapered in diameter over at least one of said sub-sectors,in the next repeat of said sequence of steps (a) to (g), theestablishment of the angle marker is effected so that the angle markeris effectively moved generally lengthwise of its last precedingposition.
 5. A method as defined in claim 4, in which said B-mode imageis displayed on a screen of a television set or cathode rayoscilloscope.
 6. A method as defined in claim 4, including theadditional step of performing a Doppler frequency shift measurement inrespect of fluid flowing through said vessel, and using the establishedorientation and cross-sectional dimensions of said vessel to calculatethe flow rate of said fluid.
 7. A method as defined in claim 1 or claim2, including the additional step of repeating steps (a) to (g) and usingthe information obtained from step (e) of the last preceding sequence ofsteps (a) to (g) to perform the step of establishing the angle marker sothat the angle marker is positioned substantially on said axis of saidvessel.
 8. A method as defined in claim 7, in which, when step (e)provides information showing that the B-mode image of said vesselindicates that said vessel is non-linearly tapered in diameter over atleast one of said sub-sectors, in the next repeat of said sequence ofsteps (a) to (g), the establishment of the angle marker is effected sothat the angle marker is effectively moved generally lengthwise of itslast preceding position.
 9. A method as defined in claim 1, in whichsaid B-mode image is displayed on a screen of a television set orcathode ray oscilloscope.
 10. A method as defined in claim 1, includingthe additional step of performing a Doppler frequency shift measurementin respect of fluid flowing through said vessel, and using theestablished orientation and cross-sectional dimensions of said vessel tocalculate the flow rate of said fluid.
 11. Apparatus for establishingthe orientation and cross-sectional dimensions of a vessel having anaxis, said apparatus comprising:(a) conventional apparatus forgenerating a beam of ultrasonic energy and scanning said beam over atleast part of said vessel, to obtain a B-mode ultrasonic echogram imageof said vessel or part thereof from a number of A-mode ultrasonicimages, each corresponding to a respective A-mode line of sight of saidbeam; (b) operator-activated means for establishing an angle marker inthe form of a short line segment within or close to the B-mode image ofsaid vessel or part thereof; (c) accumulation means for accumulatingechoes relating to respective sub-sectors of a plurality of sub-sectorsof said B-mode image, said sub-sectors (i) being adjacent to each other,(ii) being intersected by said angle marker, and (iii) containingsubstantially equal numbers of adjacent A-mode lines of sight of saidscanning beam; (d) averaging means for producing a respective averageA-mode signal for each of said sub-sectors; (e) comparison means forcomparing said average A-mode signals and for obtaining differencesbetween the average A-mode signals, to enable the orientation of saidaxis relative to said angle marker to be computed; (f) combining meansfor combining said average A-mode signals to produce a composite A-modesignal; and (g) analytical means to perform edge analysis on saidcomposite A-mode signal and to obtain therefrom a value of the diameterof said vessel or part thereof.
 12. Apparatus as defined in claim 11, inwhich said accumulation means, averaging means, comparison means,combining means and analytical means are constituted by a singlecomputer programmed to sequentially perform the functions of saidaccumulation means, averaging means, comparison means, combining meansand analytical means.
 13. Apparatus as defined in claim 12, includingDoppler frequency shift measuring means for measuring the Dopplerfrequency shift of ultrasonic signals due to the flow of liquid throughsaid vessel.
 14. Apparatus as defined in claim 11, including a screenfor display of said B-mode echogram image.
 15. Apparatus as defined inclaim 11, including Doppler frequency shift measuring means formeasuring the Doppler frequency shift of ultrasonic signals due to theflow of liquid through said vessel.